This invention relates to refigeration systems and particularly to reciprocating compressors used in such systems.
A Stirling cryogenic refrigeration system includes a reciprocating compressor which provides a sinusoidal variation in pressure in a working volume of gas. The working volume includes the head space above the compressor piston and the internal voids in a refrigerator cold finger. In the case of a split Sterling system, the working volume also includes a gas line interconnecting the above spaces. Within the cold finger, a displacer moves in timed relationship with the sinusoidal pressure variations to provide cooling at one end of the cold finger.
The usual Stirling cycle includes an electric-motor-driven compressor, and the invention will be described with reference to a split Stirling cycle of that type.
One problem encountered in Stirling sysytems is that there is always some leakage past the dynamic seal between the reciprocating compressor piston and its cylinder. The leakage is between the working volume and a control volume on the opposite side of the piston. Where there is a greater leakage in one direction than in the other movement of the piston, the mean value of the pressure in the working volume of gas tends to change relative to the gas pressure of the control volume.
One type seal which might be used in such compressors is a U-cross-section annular seal seated in a groove surrounding the piston. Such a seal advantageously expands with a pressure differential in one direction to thus increase the sealing action of the seal. Unfortunately, the direction of greater leakage past such a seal often changes during the life of the seal such that great fluctuations in the mean value of the sinusoidal pressure curve occur and render the compressor system unreliable.
An object of this invention is to provide a Stirling refrigeration system, and a compressor for use therein, wherein the mean value of the sinusoidal pressure variations in the compressed working volume of gas is stabilized to a predetermined level.